Low-cost sensors for the quantitative corrosion monitoring of engineering structures and equipment in the fields and industrial processes are highly desirable tools for corrosion engineers and plant operators, to enable them to effectively manage corrosion. These structures and equipment include pipelines (both external and internal), concrete re-enforcements (re-bars), airplanes, vehicles, bridge structures (including suspension cables), and equipment used in oil and gas fields, and chemical or power plants. Many of the structures and equipment are covered with protective coatings, and corrosion only takes place in isolated areas on these coated surfaces. An effect monitoring program requires the deployment of a massive number of sensors installed in critical areas or sensors that provide extensive coverage for large equipment. Another desired requirement for the sensors is the ability to provide the corrosion rate information for the actual equipment. Existing probes, such as electrical resistance (ER) probes [see L. Yang et al., “An On-Line Electrical Resistance Corrosion Monitor for Studying Flow Assisted Corrosion of Carbon Steel under High-Temperature and High-Pressure Conditions.” CORROSION/1999, Paper 459 (Houston, Tex.: NACE International, 1999).], for general corrosion, and coupled multielectrode array sensor (CMAS) probes, for localized corrosion (see U.S. Pat. No. 6,683,463), are excellent tools for real-time corrosion monitoring. But the CMAS probes are generally expensive and the ER probe has limited life and requires frequent changes of the sensing element. In addition, the ER probes measure the electrical resistance changes due to the cumulative corrosion-induced metal thinning and, thus, have slow responses. The linear polarization resistance (LPR) method [See J. R. Scully, “The Polarization Resistance Method for Determination of Instantaneous Corrosion Rates: A Review,” CORROSION/1998, paper no. 304 (Houston, Tex.: NACE International, 1998).] is another widely used, online method and gives nearly instant quantitative corrosion rates for general corrosion. However, this method is derived on the assumption that the corrosion processes are controlled by activation processes. In cases where the corrosion processes are controlled by both activation and diffusion, the LPR method may not be applicable.
Furthermore, all of the above three methods measure the corrosion rate of the probe sensing element that is made with a metal closely matching the system component to be monitored, in chemical composition and metallurgical conditions. The corrosion rates obtained from these probes are not the corrosion rate of the actual system components.
Non-destructive evaluation methods, such as the ultrasonic method and the electrical resistance field method (called field signature method by some suppliers), have been used to directly monitor the rate of corrosion that takes place on the actual pipe or equipment walls. Because the ultrasonic probe has a relatively low resolution limit (>10 μm), and the electrical resistance field method is based on the ER probe principle, both of these two methods are slow and neither can provide adequate information for day-to-day operations in a plant or a field.
The present invention is related to a method that is low cost and can be employed in large numbers in critical areas for the corrosion surveillance. This method uses a non-corroding sensing element, so that the sensing element itself does not require replacement. This method provides a bounding rate for the corrosion taking place on the actual system components, including those under degraded coatings, buried in soils or embed in concrete. In addition, the present invention provides a calibration method that allows the estimation of the corrosion rate of the actual system components based on the bounding value measurements.